+++ /dev/null
-From 4a8056dfeef49b306ad6af24a5563d7d6867aae0 Mon Sep 17 00:00:00 2001
-From: Eric Dumazet <edumazet@google.com>
-Date: Sat, 12 May 2012 03:32:13 +0000
-Subject: [PATCH] codel: use Newton method instead of sqrt() and divides
-
-commit 536edd67109df5e0cdb2c4ee759e9bade7976367 upstream.
-
-As Van pointed out, interval/sqrt(count) can be implemented using
-multiplies only.
-
-http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
-
-This patch implements the Newton method and reciprocal divide.
-
-Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
-kernel).
-
-There is a small 'error' for count values < 5, but we don't really care.
-
-I reuse a hole in struct codel_vars :
- - pack the dropping boolean into one bit
- - use 31bit to store the reciprocal value of sqrt(count).
-
-Suggested-by: Van Jacobson <van@pollere.net>
-Signed-off-by: Eric Dumazet <edumazet@google.com>
-Cc: Dave Taht <dave.taht@bufferbloat.net>
-Cc: Kathleen Nichols <nichols@pollere.com>
-Cc: Tom Herbert <therbert@google.com>
-Cc: Matt Mathis <mattmathis@google.com>
-Cc: Yuchung Cheng <ycheng@google.com>
-Cc: Nandita Dukkipati <nanditad@google.com>
-Cc: Stephen Hemminger <shemminger@vyatta.com>
-Signed-off-by: David S. Miller <davem@davemloft.net>
----
- include/net/codel.h | 68 ++++++++++++++++++++++++++++-----------------------
- 1 file changed, 37 insertions(+), 31 deletions(-)
-
---- a/include/net/codel.h
-+++ b/include/net/codel.h
-@@ -46,6 +46,7 @@
- #include <linux/skbuff.h>
- #include <net/pkt_sched.h>
- #include <net/inet_ecn.h>
-+#include <linux/reciprocal_div.h>
-
- /* Controlling Queue Delay (CoDel) algorithm
- * =========================================
-@@ -123,6 +124,7 @@ struct codel_params {
- * entered dropping state
- * @lastcount: count at entry to dropping state
- * @dropping: set to true if in dropping state
-+ * @rec_inv_sqrt: reciprocal value of sqrt(count) >> 1
- * @first_above_time: when we went (or will go) continuously above target
- * for interval
- * @drop_next: time to drop next packet, or when we dropped last
-@@ -131,7 +133,8 @@ struct codel_params {
- struct codel_vars {
- u32 count;
- u32 lastcount;
-- bool dropping;
-+ bool dropping:1;
-+ u32 rec_inv_sqrt:31;
- codel_time_t first_above_time;
- codel_time_t drop_next;
- codel_time_t ldelay;
-@@ -158,11 +161,7 @@ static void codel_params_init(struct cod
-
- static void codel_vars_init(struct codel_vars *vars)
- {
-- vars->drop_next = 0;
-- vars->first_above_time = 0;
-- vars->dropping = false; /* exit dropping state */
-- vars->count = 0;
-- vars->lastcount = 0;
-+ memset(vars, 0, sizeof(*vars));
- }
-
- static void codel_stats_init(struct codel_stats *stats)
-@@ -170,38 +169,37 @@ static void codel_stats_init(struct code
- stats->maxpacket = 256;
- }
-
--/* return interval/sqrt(x) with good precision
-- * relies on int_sqrt(unsigned long x) kernel implementation
-+/*
-+ * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
-+ * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
-+ *
-+ * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
- */
--static u32 codel_inv_sqrt(u32 _interval, u32 _x)
-+static void codel_Newton_step(struct codel_vars *vars)
- {
-- u64 interval = _interval;
-- unsigned long x = _x;
--
-- /* Scale operands for max precision */
-+ u32 invsqrt = vars->rec_inv_sqrt;
-+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
-+ u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
-
--#if BITS_PER_LONG == 64
-- x <<= 32; /* On 64bit arches, we can prescale x by 32bits */
-- interval <<= 16;
--#endif
-+ val = (val * invsqrt) >> 32;
-
-- while (x < (1UL << (BITS_PER_LONG - 2))) {
-- x <<= 2;
-- interval <<= 1;
-- }
-- do_div(interval, int_sqrt(x));
-- return (u32)interval;
-+ vars->rec_inv_sqrt = val;
- }
-
-+/*
-+ * CoDel control_law is t + interval/sqrt(count)
-+ * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
-+ * both sqrt() and divide operation.
-+ */
- static codel_time_t codel_control_law(codel_time_t t,
- codel_time_t interval,
-- u32 count)
-+ u32 rec_inv_sqrt)
- {
-- return t + codel_inv_sqrt(interval, count);
-+ return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
- }
-
-
--static bool codel_should_drop(struct sk_buff *skb,
-+static bool codel_should_drop(const struct sk_buff *skb,
- unsigned int *backlog,
- struct codel_vars *vars,
- struct codel_params *params,
-@@ -274,14 +272,16 @@ static struct sk_buff *codel_dequeue(str
- */
- while (vars->dropping &&
- codel_time_after_eq(now, vars->drop_next)) {
-- if (++vars->count == 0) /* avoid zero divides */
-- vars->count = ~0U;
-+ vars->count++; /* dont care of possible wrap
-+ * since there is no more divide
-+ */
-+ codel_Newton_step(vars);
- if (params->ecn && INET_ECN_set_ce(skb)) {
- stats->ecn_mark++;
- vars->drop_next =
- codel_control_law(vars->drop_next,
- params->interval,
-- vars->count);
-+ vars->rec_inv_sqrt);
- goto end;
- }
- qdisc_drop(skb, sch);
-@@ -296,7 +296,7 @@ static struct sk_buff *codel_dequeue(str
- vars->drop_next =
- codel_control_law(vars->drop_next,
- params->interval,
-- vars->count);
-+ vars->rec_inv_sqrt);
- }
- }
- }
-@@ -319,12 +319,18 @@ static struct sk_buff *codel_dequeue(str
- if (codel_time_before(now - vars->drop_next,
- 16 * params->interval)) {
- vars->count = (vars->count - vars->lastcount) | 1;
-+ /* we dont care if rec_inv_sqrt approximation
-+ * is not very precise :
-+ * Next Newton steps will correct it quadratically.
-+ */
-+ codel_Newton_step(vars);
- } else {
- vars->count = 1;
-+ vars->rec_inv_sqrt = 0x7fffffff;
- }
- vars->lastcount = vars->count;
- vars->drop_next = codel_control_law(now, params->interval,
-- vars->count);
-+ vars->rec_inv_sqrt);
- }
- end:
- return skb;
projects / openwrt / svn-archive / archive.git / blobdiff